Fuel cells combine hydrogen and oxygen without combustion to form water and to produce direct current electric power. The process can be described as electrolysis in reverse. Fuel cells have potential for stationary and portable power applications; however, the commercial viability of fuel cells for power generation in stationary and portable applications depends upon solving a number of manufacturing, cost, and durability problems.
Electrochemical fuel cells convert fuel and an oxidant to electricity and a reaction product. A typical fuel cell consists of a membrane and two electrodes, called a cathode and an anode. The membrane is sandwiched between the cathode and anode. Fuel, in the form of hydrogen, is supplied to the anode, where a catalyst, such as platinum and its alloys, catalyzes the following reaction: 2H2→4H++4e−.
At the anode, hydrogen separates into hydrogen ions (protons) and electrons. The protons migrate from the anode through the membrane to the cathode. The electrons migrate from the anode through an external circuit in the form of electricity. An oxidant, in the form of oxygen or oxygen containing air, is supplied to the cathode, where it reacts with the hydrogen ions that have crossed the membrane and with the electrons from the external circuit to form liquid water as the reaction product. The reaction is typically catalyzed by the platinum metal family. The reaction at the cathode occurs as follows: O2+4H++4e−→2H2O.
The successful conversion of chemical energy into electrical energy in a primitive fuel cell was first demonstrated over 160 years ago. However, in spite of the attractive system efficiencies and environmental benefits associated with fuel-cell technology, it has proven difficult to develop the early scientific experiments into commercially viable industrial products. Problems have often been associated with lack of appropriate materials that would enable the cost and efficiency of electricity production to compete with existing power technology.
Polymer electrolyte fuel cells have improved significantly in the past few years both with respect to efficiency and with respect to practical fuel cell design. Some prototypes of fuel-cell replacements for portable batteries and for automobile batteries have been demonstrated. However, problems associated with the cost, activity, and stability of the electrocatalyst are major concerns in the development of the polymer electrolyte fuel cell. For example, platinum (Pt)-based catalysts are the most successful catalysts for fuel cell and other catalytic applications. Unfortunately, the high cost and scarcity of platinum has limited the use of this material in large-scale applications.
In addition poisoning at the anode by carbon monoxide has been a problematic with the use of platinum. On the cathode side, usually more higher catalyst levels have been desired because methanol and other carbon containing fuel passing through the membrane react with oxygen on the cathode under catalytic effect of platinum thereby decreasing the efficiency of the full cell.
To improve the catalytic efficiency and reduce the cost, other noble metals and non-noble metals are used to form Pt alloy as catalysts. The noble metals include Pd, Rh, Ir, Ru, Os, Au, etc have been investigated. The non-noble metals including Sn, W, Cr, Mn, Fe, Co, Ni, Cu, etc (U.S. Pat. No. 6,562,499) has also been tried. Different Pt-alloys were disclosed as catalysts for fuel cell application. Binary Alloys as catalysts include Pt—Cr (U.S. Pat. No. 4,316,944), Pt—V (U.S. Pat. No. 4,202,934), Pt—Ta (U.S. Pat. No. 5,183,713), Pt—Cu (U.S. Pat. No. 4,716,087), Pt—Ru (U.S. Pat. No. 6,007,934), Pt—Y (U.S. Pat. No. 4,031,291) etc. Ternary alloys as catalysts include Pt—Ru—Os (U.S. Pat. 5,856,036), Pt—Ni—Co, Pt—Cr—C, Pt—Cr—Ce (U.S. Pat. No. 5,079,107), Pt—Co—Cr (U.S. Pat. No. 4,711,829), Pt—Fe—Co (U.S. Pat. No. 4,794,054), Pt—Ru—Ni (U.S. Pat. No. 6,517,965), Pt—Ga—Cr, Co, Ni (U.S. Pat. No. 4,880,711), Pt—Co—Cr (U.S. Pat. No. 4,447,506), etc. Quaternary Alloys as catalysts includes Pt—Ni—Co—Mn (U.S. Pat. No. 5,225,391), Pt—Fe—Co—Cu (U.S. Pat. No. 5,024,905), etc. On anode side, Ru plays an important role to reduce the poison problem (Journal of The Electrochemical Society, (149 (7) A862-A867, 2002) (U.S. Pat. No. 6,339,038). Ru has the ability to form OHads from water. This allows the catalytic desorption of CO as CO2. On the cathode side, non-noble metal complex catalysts, such as Fe,Co, Ni porphyrins have been utilized (Solid State Ionics 148 (2002) 591-599).
In the design of electrodes, a three-phase boundary of reaction gases (H2 and O2), catalysts and conductors (for proton and electron) is commonly required for the electro-chemical reaction. An extensively used approach to fuel cell fabrication is the so-called “ink” coating method. In this method, catalyst particles (e.g., 2-4 nm) are supported on carbon particles (15 nm of Vulcan XC72). These particles are mixed with a solution of polymer electrolyte as an ink, which is smeared on the surface of a conductor, such as carbon paper, to form a three-phase coating. In this approach, an electrolyte film covers the mixed particles of catalyst and carbon. Therefore, no direct three-phase boundary exists in this structure. Reaction gases, H2 and O2 do not directly contact the catalyst, but rather, must diffuse through the electrolyte layer to reach the catalyst surface. On the cathode side, protons must diffuse through the electrolyte layer to reach O2-ions. Therefore, there exists two opposite requirements: Protons need a thick electrolyte layer to maintain good conductivity. On the other hand, a thick electrolyte layer forms a diffusion barrier for O2. To solve this difficulty, some improvements have been suggested for the “ink” coating design. Toyota company (in U.S. Pat. No. 6,015,635) suggested the use of pure electrolyte clusters inserted into the “ink” coating layer to increase proton conductivity. In U.S. Pat. No. 6,309,772), it is suggested that electrolyte coated and un-coated carbon-catalyst particles are mixed to form the “ink” layer to improve gas diffusion. In these “ink” coating structure, the efficiency of the catalysts are still restricted by gas and proton diffusion.
More recently some new catalyst structures were used to increase the catalytic efficiency. For example, 3M Company (U.S. Pat. Nos. 5,879,827 and 6,040,077) used a nanostructure electrode. In this structure, an acicular nano polymer whisker supports deposited acicular nanoscopic catalytic particles. At first, an organic material is deposited on a substrate. Then the deposited layer is annealed in vacuum, and forms a dense array of acicular nano polymer whiskers. The preferred length of the whiskers is equal or less than 1 micrometer. Then, catalyst thin film is deposited on the supporting whiskers. The diameter of catalyst particle is less than 10 nm, and the length is less than 50 nm. In a Pt and Ru loading range of 0.09-0.425 mg/cm2, the fuel cell obtained a satisfactory catalytic efficiency. However the process is complicated by non-electrical conducting nano polymer whiskers and transferring the catalyst coated polymer whisker layers onto carbon electrodes. Pt mixed carbon inks are still used under the whisker layer in this design.
Gore Enterprise Holdings (U.S. Pat. Nos. 6,287,717 and 6,300,000) used a direct catalyst thin film coating on carbon electrodes or on Pt mixed carbon ink layers. The catalyst thin film played an important role as an interface layer which could have a different platinum concentration than the rest of catalyst layers. This structure effectively reduced the platinum contents of the catalyst used in the fuel cells. A catalyst loading less than 0.1 mg/cm2 was claimed.